1. Field of the Invention
The present invention relates to a torque sensor of noncontact type and, more particularly, to a torque sensor for converting the torque of a shaft into an electric signal and to an electric machine in which the torque sensor is mounted.
2. Discussion of Background
Recently, systems for generating torque, e.g., an engine system, are required to detect a torque and produce a detection signal corresponding to the detected torque for controlling the torque generation system. Various torque sensors for detecting a torque of a shaft have been developed to meet the demand noted above. Particularly, there have been proposed noncontact torque sensors, which can measure the torque of a shaft accurately in a state of noncontact with the shaft, as disclosed in William J. Fleming and Paul W. Wood, "Noncontact Miniature Torque Sensor for Automotive Application" SAE paper 8206206, presented at the Automotive Engineering Congress, K. Harada, I. Kawajiro, M. Inoue, "A Now Torque Transducer Using Stress Sensitive Amorphous Ribbon", IEEE Transaction on Magnetics, Vol. MAG-18, No. 6, November 1982, and Japanese Patent Disclosure (KOKAI) No. 57-211030 (corresponding U.S. application Ser. No. 268890 filed June, 1, 1984). The torque sensor disclosed by William J. et al. in "Noncontact Miniature Torque Sensor for Automotive Application" measures torque applied to an engine crankshaft by making use of the fact that a magnetic characteristic, i.e., magnetic permeability, of the crankshaft is varied according to the torque applied thereto. However, the magnetic property of the measurement zone of the engine crankshaft is not uniform, and there is magnetic anisotropy on the surface and the inside of the crankshaft. Therefore, the torque cannot be measured accurately. In addition, since the engine crankshaft itself does not have high magnetic permeability, it is necessary to generate a magnetic flux sufficient to penetrate the crankshaft measurement zone and be detected. Therefore, the magnetic flux control means is inevitably large in scale. In the torque sensor disclosed by K. Harada et al. in "A New Torque Transducer Using Stress Sensitive Amorphous Ribbons" and Japanese Patent Disclosure (KOKAI) No. 57-211030, an amorphous magnetic ribbon is arranged along the entire circumference of a portion of a shaft, and a coil assembly is provided around and coaxially with the shaft. The coil assembly generates a magnetic flux parallel to the axis of the shaft, and the permeability of the amorphous magnetic ribbon that is changed, according to the torque coupled to the shaft, is measured. Although this torque sensor can solve the problem noted above, since the coil assembly generates a magnetic flux parallel to the axis of the shaft, which has a relatively high magnetic reluctance, a comparatively large exciting current has to be supplied to the coil assembly. In addition, since the coil assembly is provided around the shaft, a space for providing the coil assembly is necessary around the shaft. Therefore, the torque sensor cannot be readily assembled in the system which generates the torque, and depending on the particular application, there may not be sufficient space for assembling the torque sensor. Further, the amorphous magnetic ribbon arranged along the entire circumference of the shaft has to be given an induced magnetic anisotropy in a predetermined direction. However, it is difficult to give the amorphous magnetic ribbon arranged along the entire circumference of the shaft, i.e., a cylindrical amorphous magnetic ribbon, an induced magnetic anisotrophy in a predetermined direction. Further, the magnetic permeability of the amorphous magnetic ribbon arranged along the entire circumference of a shaft may not be uniform when the shaft is made of a Fe system. Variations of the magnetic permeability are liable to result without variations of the torque while the entire circumference of the shaft is under measurement due to lack of uniformity of the magnetic property of the shaft. Therefore, noise is introduced into the torque detection output, and the signal-to-noise ratio (S/N) is reduced. For solving such problems, a torque sensor of noncontact type was developed by Kobayashi et al. as described in U.S. Pat. No. 4,590,807, which can be readily disposed in a comparatively small space and can measure the torque of a shaft with a comparatively small exciting current and also with a sufficient S/N ratio.
However, the torque sensor of U.S. Pat. No. 4,590,807 by Kobayashi et al. also uses the variations of the magnetic permeability of the amorphous magnetic ribbon.
If, the ribbon is magnetically disposed onto circumference of the electrical machines such as an induction motor or generator, the obtained S/N ratio of the detecting output of the torque is reduced under the influence of the induction magnetic flux which the machine itself produces.